This is bad news Markus, I hope you can sort it out soon. The MicroBrute is truly a great-sounding and highly compact analog synth, too bad though that the same amount of effort and care were not invested in quality control. This is a relatively new product which is available at a really good price but none of these facts can justify as an excuse for MicroBrutes going dead or malfunctioning in any way, all this too soon after buyers had started using them. I'm only saying this because I read some arguments posted here on behalf of Arturia that this is a cheap (affordably priced) analog synth and as such we should accept that cheaper = less reliable. I'm not quoting this but this is what that post was supposed to communicate. A really weird way of handling technical issues and customers, to say the least... Anyway, I would have gladly paid 100 or 150 EUR more for a more stable and reliable instrument. Mine still works OK (though I'm not sure if the mod-matrix is flawless.... some patchings seem to totally leave the sound unchanged whatsoever.. yet I may not do things correctly...I will have to experiment more with this...) but I'd lie if I said I'm not worried a little. Also, I guess I'd love to add a BeatStep to my rig but it's another affordable device so what should I expect? I'll wait for more user feedback before I decide and buy... or not buy.
In case you haven't tried yet, you may want to check if your MicroBrute powers up with a different PSU that has the same voltage and amp specs. Others have had similar 'no proper power up' issues and in many cases the PSU turned out to be defective.